The present invention relates to the transition between thermocouple leads and extension wires, and in particular to making the transition between thermocouple material leads and extension wire inside of a multi-conductor mineral insulated cable so that the transition can be made in a controlled manner, environmentally protected as close as possible to the thermocouple so that the length of the thermocouple material leads is minimal.
A thermocouple generates a voltage proportional to the junction temperature. In many applications, such as measuring a temperature distribution inside of a reactor vessel, the voltage measuring instrumentation is located outside of the reactor containment building, perhaps several hundred feet from the temperature measuring thermocouple. One known method to provide the voltage generated by a thermocouple to the processing instrumentation is to connect the thermocouple leads directly to the processing instrumentation with long runs of thermocouple leads therebetween. The thermocouple leads are made of particular, expensive alloys depending upon the type of thermocouple used. It is, therefore, commercially desirable to make a transition from thermocouple leads to a lesser expensive conductor, such as copper wire, as close to the temperature measuring thermocouple as practical.
One known technique for making the transition from thermocouple leads to a lesser expensive conductor includes terminating the leads of several temperature measuring thermocouples in a junction box within the reactor containment building with copper extension wires conducting the thermocouple generated voltage to the processing instrumentation outside the reactor containment building. The temperature inside the junction box is measured by another temperature measuring device so that the thermocouple generated voltage as measured by the processing instrumentation may be compensated for the voltage introduced at the intermediate junctions within the junction box. This technique requires thermocouple leads of substantial length be run from the temperature measuring thermocouples to a location within the containment building where it is convenient to locate a junction box. Furthermore, a temperature gradient may exist within the junction box such that all of the intermediate junctions within the junction box are not at the measured temperature of the junction box. Thus, an error is introduced in the measured temperatures due to a temperature gradient at the intermediate junctions within the junction box.
Therefore, it is an object of this invention to provide apparatus which is capable of making the transition from leads of a temperature measuring thermocouple to extension wires closer to the temperature measuring thermocouple than is practical using a junction box.
It is a further object of this invention to make the transition from leads of a temperature measuring thermocouple to extension wires in an environmentally protected and miniaturized manner which maximizes measurement accuracy by eliminating temperature gradients.